package com.zzw.hj100;

import java.util.Scanner;

/**
 * @Project: hw_java
 * @Description: 等差数列
 * @Author: zzw
 */

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        // 等差数列公式 s_n=n*a1+n*(n-1)*d/2
        int result = 2 * n + n * (n - 1) * 3 / 2;
        System.out.print(result);
    }
}